Flexible torque transmission couplings are old and well-known devices for transmitting rotary power under conditions in which misalignment exists between the rotating power source and the machine receiving the rotating power. Flexible couplings employing one or more flexible elements consisting of an integral, single piece of material or comprising separate flexible discs layered together, and other components connected together are widely used. An example of an integral, single element flexible coupling is disclosed in U.S. Pat. No. 3,071,942, issued Jan. 8, 1963. An example of a flexible coupling comprising a number of flexible layered discs and connecting members to provide an overall coupling structure is disclosed in U.S. Pat. No. 2,745,267, issued May 15, 1956.
As a result of the stresses on the flexible coupling during its operation, including stresses due to misalignment of the power source and the driven device, cracking occurs in the flexible disc layers over a period of time and ultimately the entire flexible member and thereby the flexible coupling itself fails. The failure of the coupling may result in further damage to itself as it rotates in a failed mode, thus increasing the cost of its repair. The failure may also occur at a time when continuous operation of the device driven by the power source is critical and the resulting shut-down would be extremely costly in a processing or manufacturing operation. Consequently, there is an important need for a means for detecting cracks in the flexible discs of the coupling at an early time in their occurrence before failure of the entire coupling occurs or shutdown of an entire production line is necessary.